The present invention relates to the manufacture of tactile sensors that use electrically conductive elastomer and, in particular, to the attachment of electrodes to the elastomer.
A useful type of tactile sensor is one that senses not only the presence or absence of force, but the position and amount of applied force. Tactile sensors that use an electrically conductive elastomer (referred to as “elastomer”) can convert the position or magnitude of applied force to a voltage (or current) using resistance changes. With tactile sensors having an electrically conductive elastomer, the internal resistance of the elastomer or the contact resistance between the elastomer and sensing electronics varies with the amount of applied force. Measurement of the position and of the amount of applied force can be made through proper application and measurement of voltage levels on the elastomer, which requires sufficiently rigid or firm attachment of electrodes to the elastomer in order to produce repeatable measurements. The elastomer must be linked to electronics that provide excitation (from a power source) and to devices that measure or use directly at least one electrical property. The electrodes that provide the electrical pathway are made typically from metal strips, wire, or sheets of conductor.
Besides reducing the overall number of parts and minimizing the steps required for production, the most important non-trivial problem encountered when designing and producing such a tactile sensor is the electrode connection between the electrically conductive elastomer and the electronics.
For a large group of sensors, the electrodes do not necessarily need to flex significantly and can be relatively small. For example, wire electrodes may run along an edge of a flexible elastomer sheet (such as in U.S. patent application Ser. No. 09/654,481 filed Jan. 9, 2000 and entitled “Tactile Sensor” (Attorney Docket No. 99P5844US) which is incorporated by reference herein) or the electrodes may be fixed printed paths on a rigid printed circuit board (such as in U.S. Pat. No. 5,886,615). The quality of a conductive elastomer tactile sensor is judged on its ability to measure accurately, precisely, and consistently the resistance changes via the electrodes.
The contact impedance between the electrodes and elastomer needs to be known when the sensor is not under load (or when the elastomer first touches the electrode). Fluctuations of this contact resistance will influence the sensor measurements; precision and accuracy of position and/or pressure measurements are directly related to changes in non-loaded contact resistance. This is especially true for electrically conductive elastomers that have changing internal resistance with applied pressure; these are called piezoresistive materials. When using a piezoresistive elastomer in a tactile sensor (example described in U.S. Pat. Nos. 4,322,983 and 5,510,812), the contact resistance between the electrodes and elastomer needs to be known and minimized. Fluctuations of this contact resistance will influence significantly the sensor precision and accuracy; Burgess in U.S. Pat. No. 5,060,527 shows an example of this fact.
Tactile sensors have been disclosed (e.g., GB 2222258A, DE 19750671A, and Ser. No. 09/654,481) that use electrically conductive foams (referred to as “foams”) that do not have significant internal resistance changes under load. These sensors measure the contact resistance between the electrodes and foam or the contact resistance between multiple layers of foam. In each of these cases, the reliability of the voltage application and the resistance measurements would be dramatically improved by firmly attaching at least one set of electrodes to the foam. In the case of Ser. No. 09/654,481, both sets of electrodes should be rigidly or firmly attached to the foam to produce repeatable measurements.
In the prior art, electrically conductive elastomers were connected to electronics or batteries using metal strips, wire, or sheets of conductor (different embodiments of electrodes). In most prior art the electrodes were not held rigidly to the elastomer; some sensors (e.g., U.S. Pat. Nos. 5,886,615 and 6,114,645) required the elastomer to deform before touching any electrode, which reduces significantly the sensitivity and precision of the sensor. In the other cases, the metal strips, wire, or sheets of conductor were held to the electrically conductive foam using pressure applied by external components or using particular electrically conductive adhesives (flexible adhesives with carbon, silver, or nickel mixed into it).
The prior art mentioned above suggests using pressure applied by external components to hold an electrode to electrically conductive elastomer (examples described in U.S. Pat. No. 4,322,983, DE 19750671A, and Ser. No. 09/654,481). Unfortunately, this method of linking electrode to elastomer does not have a precise (that is, a repeatable) contact resistance; given the similar loading and forcing histories, the contact resistance will vary when the sensor is not loaded. This problem is especially prevalent when the elastomer may be subject to being pulled as well as pushed.
Prior art has also suggested using conductive adhesives, such as conductive tape (3M 9713 XYZ-axis Electrically Conductive Tape) or conductive epoxies (DOW Silastic 732™ made conductive with Shawinigan Black™). These approaches have problems when used for manufacturing a sensor that uses an electrically conductive elastomer. Conductive tapes are typically designed for shielding applications that require connecting thin metal foils to grounded members. Though these tapes hold well to flat, smooth metal surfaces, they do not provide a constant contact resistance between the electrically conductive elastomer and metal electrodes. This is especially true during bending or flexing of elastomer at or near the electrode attachment; the contact resistance varies with deformation and, therefore, with forcing. Furthermore, when the elastomer takes the form of an open or closed cell foam, the tapes may not provide a good conductive path because the tape will only contact the top most foam lattice members. For this reason, conductive tapes cannot be used reliably in tactile foam sensors that measure accurately both position and pressure.
In some cases, conductive epoxies (or adhesives made to be conductive) will adhere well to and provide a reliable connection between electrodes and electrically conductive elastomer. Burgess, in U.S. Pat. No. 5,060,527, recommends using either polyvinyl chloride (PVC) or Dow Silastic 732™ made conductive with carbon, silver, or nickel. These adhesives, however, should not be used to adhere to most metals that would be used as electrodes (copper, zinc, and brass) or anything sensitive to acetic acid (Dow product information guide). Additionally, the suggested adhesives may bond well to the polyurethane or silicon elastomers recommended by Burgess, but do not bond well with most electrically conductive elastomers.
In general, the electrically conductive elastomer, e.g., electrically conductive polyethylene (Evazote™ foams made by Zotefoams) or polypropylene, may be chemically inert and difficult to bond, but such electrically conductive elastomers are preferred for some applications because they are inexpensive, biologically inert, uniform (density and resistance), and do not outgas. Special epoxies and treatments can be used to bond to electrically conductive elastomers, but such epoxies and treatments are expensive, require multiple steps, and/or contain chemicals that are harmful to the environment. Furthermore, the curing process associated with epoxies and the out-gassing that occurs with epoxies or treatments delay manufacturing and may require special chemical mixing, monitoring equipment or special environmental conditions to perform.
As seen from the above, improved methods for providing good electrode connections to electrically conductive elastomers in tactile sensors are desired.